1. Field of the Invention
The present invention relates to a method for generating a spatio-temporal field in a reverberant environment in order to analyse the behaviour of an object under test placed in the reverberant environment. It has a particularly useful application in the detection of immunity failures in objects subjected to electromagnetic emissions or even in the response of objects subjected to a particular field. The present invention can also be applied in the field of acoustics.
2. Description of the Related Art
In general, with the increase in the amount of electronic equipment in current systems, it is becoming essential to verify their susceptibility to radiated emissions.
In order to provide for the simultaneous operation of such pieces of equipment in proximity to one another, it is necessary to verify their ability to operate satisfactorily in their environment without producing any interference which is unacceptable for everything present in that environment. This leads to a need to investigate the response of an object subjected to a field, whether it be a component, a piece of equipment or a complete system, in order to determine the access routes vulnerable to external attacks and counteract such defects.
Thus, obtaining high field levels in the radiofrequency domain while still keeping the cost of measuring instruments as low as possible has become a priority for manufacturers, who are required to verify that the new products they are proposing meet the compatibility standards, requiring numerous tests.
However, these tests may be costly in terms of system downtime, which is reflected in a financial loss when a production line is started up or in terms of the instrumentation and power to be implemented for such tests.
This may result in the object under test being subjected to locally plane waves with well specified characteristics such as a direction of propagation and a polarization, as is the case for tests performed in an anechoic chamber. For this type of directive tests, the number of test configurations is quickly limited, a few directions of arrival at the object under test and two polarizations (horizontal and vertical) being used to minimize the downtime of the systems being tested, since mechanically changing the configuration takes some time. What is more, the generation of a high-intensity field requires the involvement of high power levels, which gives rise to extra cost in terms of equipment and maintenance.
Another solution often used in the prior art is the generation of intense, isotropic and homogeneous fields by means of reverberant chambers with mode mixing. These can be used simultaneously to excite a considerable number of locally plane waves coming from random directions and with different polarizations while still ensuring a high mean field level on the basis of relatively low injected powers. However, they have disadvantages concerning the accuracy of the measurements, which are dependent on the tolerance as regards the uniformity of the field and the random nature of the directions of propagation of the field and its excited polarizations. In fact, the random nature of the attack, although capable of detecting the presence of a fault in a system, cannot be used precisely to identify the origin or the location without performing subsequent tests in an anechoic chamber.
Document WO 2005/104473 A1 is known, which describes the technique of time reversal of a wave and involves the transmission of signals in accordance with a wireless point-to-point communication model with no ability to check the directivity or polarization of the propagation of the wave front.
The purpose of the present invention is to remedy the above-mentioned disadvantages by proposing a method for generating an intense field and checking its directivity, direction of arrival and polarization. Another purpose of the invention is real-time checking of the spatio-temporal distribution of the field produced.
At least one of the above-mentioned objectives is achieved with a method for generating a spatio-temporal field in a reverberant environment in order to analyse the behaviour of an object under test placed in the reverberant environment. The method according to the invention comprises the following steps:                at least one transfer function between a first group of transducers comprising at least one transducer placed in the reverberant environment and a second group of transducers comprising at least one transducer also placed in the reverberant environment is estimated,        taking account of the spatial distribution of the transducers of the first group, a spatio-temporal excitation field to be applied to the object is considered,        primary signals which, when applied to the first group of transducers, make it possible to obtain said spatio-temporal excitation field are modelled,        said at least one transfer function is applied to these primary signals so as to obtain secondary signals,        a technique known as time reversal in the time domain or phase conjugation in the frequency domain is used on these secondary signals so as to determine excitation signals, and        these excitation signals are applied to the transducers of the second group so as to obtain said spatio-temporal excitation field on the object.        
The reverberant environment may be a reverberant chamber or an enclosure with a high reflectivity of the walls to incident waves.
Thus, the first group of transducers is used to estimate the transfer functions. On the other hand, this first group is no longer needed during the injection of excitation signals, i.e., for example, during measurements of the electromagnetic compatibility of an object under test. In fact, once the transfer functions have been determined, numerous wave propagation scenarios can be implemented by modifying the excitation signals only, the position of the transducers of the second group remaining identical to their position during estimation.
The method according to the invention involves a learning phase for the initial wave path and focusing of the excitation wave solely on an emission source, as in the prior art. The present invention makes it possible to generate coherent, high-intensity and polarized spatial distributions of the spatio-temporal field in a reverberant environment while still retaining the ability to focus this field anywhere in the reverberant environment.
The invention has a serious advantage with respect to the prior art with regard to the duration of the tests, which is only limited by the physical limits of the test environment, such as, for example, the propagation time of the wave front generated. Thus, a representative test duration of 1 ms allows for 1000 different configurations in one second on the basis of one single initial characterization phase (obtaining transfer functions for a given positioning of the transducers).
According to an advantageous characteristic of the invention, during the modelling stage, each primary signal is defined in the form of αj.X, where X is the frequency spectrum of a signal as a function of the time variation to be obtained; α is a complex coefficient of amplitude and phase weighting for the signal X, and j is an index denoting each transducer of the first group of transducers, these coefficients αj being defined as a function of the directivity, and/or direction of arrival and/or intensity of the spatio-temporal excitation field and/or the polarization.
Preferably, the αj are defined in accordance with a technique for synthesizing a network of transducers in free space. More precisely, during estimation of the transfer function, there is a time window where the radiation from the transducers of the first group is propagated under free space conditions during which the wave front emitted has not interacted with the walls of the reverberant environment. The time reversal technique makes it possible to find this time window by transforming the divergent wave front into a wave front converging towards the object under test.